Implantable medical devices, such as stents, heart valves, bone plates, intrauterine contraceptive devices and the like must meet many requirements to be useful and safe for their intended purpose. For example, they must be chemically and biologically inert to living tissue and to be able to stay in position over extended periods of time. Furthermore, devices of the kind mentioned above must have the ability to expand from a contracted state, which facilitates insertion into body conduits or cavity, to a useful expanded diameter. This expansion is either accomplished by a forced expansion, such as in the case of certain kinds of stent by the action of a balloon-ended catheter, or by self-expansion such as by shape-memory effects.
A widely used metal alloy for such applications is the nickel-titanium alloy, known as "nitinol". Under certain conditions, nitinols can be highly elastic such that they are able to undergo extensive deformation and yet return to their original shape. Furthermore, nitinols possess shape memory properties such that they can "remember" a specific shape imposed during a particular heat treatment and can return to that imposed shape under certain conditions.
The shape memory effect of nitinols results from metallurgical phase transformations. Certain nitinols are characterized by a transition temperature or transition temperature range, above which the predominant metallurgical phase is termed "austenite" and below which the predominant metallurgical phase is termed "martensite". The transformation temperature from austenite (or austenitic state) to martensite (or martensitic state) is termed "martensitic transformation"; the reverse transformation from austensite to martensite is termed as "austenitic transformation". The transformations occur over a range of temperatures and are commonly discussed with reference to M.sub.s and M.sub.f, the start and finish temperatures of the martensitic transformation, respectively, and A.sub.s and A.sub.f, the start and finish temperatures of the austenitic transformation, respectively. Transformation between these two phases is reversible such that the alloys may be treated to assume different shapes or configurations in the two phases and can reversibly switch between one shape to another when transformed from one phase to the other. In the case of nitinol medical devices, it is preferable that they remain in the austenitic state while deployed in the body as nitinol austenite is stronger and less deformable and thus more resistant to external forces as compared to nitinol martensite.
Implantable medical devices made of nitinol have been known in the art. See for example U.S. Pat. Nos. 3,786,806, 4,485,816 and 5,037,427. In U.S. Pat. No. 5,562,641, a two-way shape memory effect is employed such that the austenitic transformation temperature is above body temperature and the martensitic transformation temperature is below body temperature, whereby the device retains its last conditioned state (e.g. austenite or martensite) at body temperature. U.S. Pat. No. 5,624,508 discloses a method for the manufacture of shape memory alloy (SMA) device with defined transformation temperature. In many such devices, A.sub.s is considerably above body temperature and accordingly for converting the device into the austenitic state, it is necessary to provide heat in an extent which in addition to being difficult to apply may be damaging to the surrounding tissue. In devices where A.sub.s is only slightly above body temperature, the austenite may become destabilized, e.g. as a result of a stress-induced martensitic transformation, rendering the device less resistant to external stresses.
In many conventional nitinol medical devices, there is often a large temperature range between A.sub.s and A.sub.f, which thus makes it difficult to establish, in an accurate and reproducible manner, the extent of the austenitic transformation upon heating.
The use of stress-induced martensite principle, rather than temperature-induced martensite, has likewise been employed in medical devices, e.g. in U.S. Pat. No. 4,665,906. In such devices, austenitic nitinol is deformed to form stress-induced martensite and held in its deformed configuration and martensitic state by a restraining member. The device is introduced into the body in the deformed configuration, where it is removed from the restraining member to return to its austenitic state and configuration without any temperature change. In the case of such a device a restraining member has to be employed and once the medical device is released from the restraining member, it is almost instantly deployed. If the device is not accurately positioned immediately before release from the restraining member, it may have to be removed with some damage to the surrounding tissue.